Instrument for determining vitamin a deficiency



March 11, 1941.

MILLl-FOOT CANDLES MI LLl-FOQT' CANDLES MILLI-FOOT CANDLES w. 0.,FROHRING EI'AL 2,234,240 INSTRUMENT FOR DETERMINING VITAMIN DEFICIENCYFiled Oct. 26, 1937 4 Sheets-Shet 1 IN ,MINU'I ES IO I5 20 TIME TIME p NMINUTES TIME IN MINUTES I. l 4 2 F1 70 E .5

IMIN. 2mm. SMINUTE 'AMINUTE 5% MINUTE mum on .0

Ll I 6c 6' INVENTORS illiam O. Frohrin Howard I. Seiberl;

BYJosePh M. Tabor w. 0. FRCHRlNG EIAL March 11, 1941.

(Sheets-Sheet 2 Filed Oct. 26,1937

11111111 r glllllhflrllllllllll INSTRUMENT FOR DETERMINING VITAMIN ADEFICIENCY IIIIIlIIIIIIIIIIII/I. VII/III!!!fllllllllIIIIIIIIIIIIIIIIIIIIIIIIIIL 'IIIIIIIIIIIIIIIIlIIIIIII/IIIL '/I/INVENTORS William O- Frohrin q Howard E Seibert Joseph M. Tabor BYM44495 12 ATTORNEYS inn Mach 4 w. o. FROHRING z-z-r' AL r 2,234,240

INSTRUMENT FOR DETERMINING YITAMIN A DEFICIENCY I Filed 0015. 26, 1937 4Sheets-Sheet 5 INVENTORS W'iLliarn 0-Fr0hrin3 Howard E Seibert Joseph M.Tabor- ATTORNEYS March 121, 1941.v w, OQFR O HRING ETAL 2,234,240

INSTRUMENT FOR DETERMINING VITAMIN A DEFICIENCY 26, 1937 4 Sheets-Sheet4 ll m 0 1 V INVENTORS William O. Frohriny Howard F Seibert YJoseph MTaber 1 ATTORNEYS Patented Mar. ll, 3%41 perish stares arem series.

KNSTRUMEN'E FOR BEfiRRElINHNG VETIAMI A DEFECHENQY Application October26, 1937, Serial No. l? 1,089

3 (Claims.

Our invention relates to an improved method of and to an instrument fortesting or examining patients to determine their vitamin A status and isacontinuation in part of our application Ser.

55 No. 85,128 filed on June 13, 1936. More particularly, it relates to ascientific instrument and to a method or examining the human eye todetermine the rate at which visual purple is depleted in the retina ofthe eye in comparison to that 9 of a normal individual bya source orbright light which is capable oi depleting visual purple at a ratefaster than it is regenerated.

The correlation between night blindness and vitamin A deficiency is nowwell known. For instance, a recent report of the Journal of the AmericanMedical Association, May it, 1936, page 1733, states that It isgenerally agreed that the physical symptom of vitamin A deficiency isnight blindness, or nyctalopia." Night blindness is caused by theinability of an individual to regenerate quickly a sumcient amount orvisual purple in the retina of the eye to see in a iai tly lighted spaceafter the eye has been subjected to bright lights, such as when walkingfrom a brightly lighted lobby into a dark or semi-dark theater. Thedifiiculty in seeing is attributed to the fact that the visual purple inthe retina of the eye has been depleted by bright lights but after anindividual has been in the theater for a short period, a sufdcieritamount of visual purple is regenerated in the retina of the eye toenable the individual to distinctly see the aisles and seats. Nightblindness caused by vitamin A deficiency will disappear promptlyifvitamin A is restored to the system. If vitamin A is not restored to thesystem within a reasonable length of time, however, xeropthalmladevelops, which finally leads to blindness. greater importance, however,is the fact that by our improved method of. testing 40 the eye, evenmild degrees of vitamin A deficiency may be readily determined andvitamin A therapy may be, instituted and the condition corrected.

It is now well recognized that bright light bleaches the visual purplein the retina. of the eye and that its subsequent replenishment isdependent upon the amountot vitamin A in the system. Previousinvestigators have therefore 'soughtto test patientsto determine theirvitamin Astatus by first bleaching the visual purple and thendetermining the rapidity with which it is replenished as compared tothat of a normal person after the eye has been dark adapted for apredetermined period of time. Such methods, however, have failed-to takeinto consideration that during the bleaching of the visual purple in theretina (ct. co et) of. the eye, it is being constantly regenerated. Inour investigations of this subject we have also found that the lightenvironment to which the patient was subjected before the bleachingoperation and the quality of light utilized in the 5 bleaching andtesting of the eye are material factors in determining vitamin Adeficiency.

It is therefore the aim of our invention to provide an improved processof examining the human eye to determine the vitamin A status ofindividuals or patients being examined.

Another object of our invention is to provide an improved process oftesting the human eye to determine the degree of visual purple depletioneffected in the retina of the eye when it is ex- 15 posed to a standardsource of bright light after a-predetermined dark adaptationperiod ascompared to the visual purple depletion efiecte'd in the retina oitheeye of a normal person subjected to the same treatment. 2

A further object of our invention is to provide an improved self-contained instrument including all the necessary parts and apparatus tocomplete an examination of a. patient and determine his ability ascompared with the ability of a normal person to regenerate visual purplein the retina of the eye while it is being subjected to a source ofbright light of sumcient intensity to deplete visual purple in theretina of the eye faster than it is regenerated.

Another object of the invention is to provide an improved instrument ofsimple construction by means of which vitamin A deficiency ofindividuals may be readily indicated.

Further objects of our invention will be apparent as the descriptionproceeds.

Our invention will be better understood by ref; erence to the accompanyng drawings, in which Figs. 1, 2 and 3 are charts illustrating ourimproved method. of testing the eye to determine the vitamin A status ofindividual or patients and showing the eflect or vitamin A therapy;

Fig. 4- is a chart showing the efl'ect oi! previous light environmentupon the rate of the regeneration of visual purple in the dark adaptedeye;

Fig. 5 is a side elevation of our improved instrument;

Fig. 6 is a. longitudinal sectional view showing the quincunx in idleposition; 50 Fig. 'l is a cross sectional view on the line |1 of Fig. 6;

Fig. 8 is a. cross sectional view on the line 8-8 of- Fig. '1;

Fig. 9-1.; a detail viewor the neck portion or the 55 instrument showingthe quincunx in operative 2 showing the threshold to light of the samein- 50 1 test light may be of any predetermined quality,

position;

- Fig. 10 is a cross sectional view on the line |0l0 of Fig.6;

Fig. 11 is a bottom plan view of the instrument, the bottom portionbeing removed;

Fig. 12 is a view on the line' l2--l2' of Fig. 9. showing one suitableway in which varying intensity of the light through different aperturesin the quincunx may be secured;

Fig. 13 is a diagrammatic view showing the electrical connections forthe apparatus;

Fig. 14is'a view of amodified form of an in strument by means of whichan approximation of the vitamin A status of individuals may be obtainedby our improved method;

Fig. 15 is a rear view of the apparatus shown in Fig. 14;

Fig. 16 is a digrammatic view of the electrical connections for theapparatus shown in Fig. 14;

Fig. 17 is a cross sectional view on the line ll-l'l of Fig. 14;

Fig. 18 is an enlarged elevational view of the apertured panel shown inFig. 14;

Fig. 19 is a cross sectional view on the line l9l9 of Fig. 18; and

Fig. 20 is a-modified view showing another arrangement of forming thenumerals shown in Fig. 18'. w

Referring first to Figs. 1 to 3, inclusive, of the drawings in which theordinate represents millifoot candles and the abscissa'designates timein minutes, chart's are given showing difierent individuals who weretested by our improved process, both before and after vitamin therapyhad been instituted.

As shown in Fig. 1, when the patient enters the room the thresholdof.-his eyes to light is first determined as indicated by the letter Kin curve I. The eyes are then dark adapted for a sufiicient length oftime to permit the visual purple in the retina of the eye to becomesubstantially constant and the threshold to light is again taken at theend of that period or at a predetermined interval or intervals 1 of timeduring and at the end of that period; for instance, the threshold tolight may be taken at the end of a five minute period, as indicated at Yon the drawings, and again at the end of a ten minute period, asindicated at Z on the drawings. While the a small Mazda bulb providedwith a daylight filter connected in series with a potentiometer forvarying the quantity of light was utilized in preparing the charts. Theeyes were then bleached tor a predetermined period of time with a lightof sufiicient intensity to bleach the visual purple in the retina of theeye of a normal person faster than it is regenerated. It is essential to"provide a bleaching light which is of the same eyes were thendarkadapted and the threshold to light was determined by the small testlight after a predetermined period or at intervals during-and at the endof a predetermined period,

As illustrated in the chart, the dark adapted eye was tested at three,five and ten minute intervals as indicated at points E, F and G on thechart.

Vitamin A therapy was then instituted, curve dividual under the sameconditions after being administered 30,000 U. S. P. units of vitamin Aconcentrate daily for five days, and curve 3 his threshold to lightafter being administered 30,000 U. S. P. units of vitamin A daily for aperiod of thirty days. The improvement in the individuals threshold tolight will be readily apparent from the chart. From an examination of alarge number of people, the chart of the individual shown in Fig. '1indicates that his vitamin A status ranges from normal to slightlydeficient.

Thermeasurements of the threshold to light of the individual whose chartis shown in Fig. 2 were taken in the same manner. The chart,

however, shows that this individual had a poor threshold to light beforevitamin A therapy was instituted, .as indicated by the significantpoints on curve la. The significant points on curve 2a, 3a and 4aindicate the threshold of light of the same individual after beingadministered 60,000 U. S. P. units of vitamin A concentrate daily for aperiod of seventeen days, twenty-four days and thirty-six days,respectively. The remarkable improvement as indicated by the significantpoints on curve 4a willbe readily apparent.

In the chart shown in Fig. 3, the significant points on curve lbindicate the threshold to light of an individual who is above normal,that is, the individual is not deficient in vitamin A. To test theeffects of vitamin A therapy upon this individual, 30,000 units ofvitamin A concentrate was administered daily for five days and tests-were again made as indicated by the significant points on curve 21) anda similar quantity of vitamin A concentrate was administered daily for aperiod of fifteen days and the significant points again determined asindicated by the curve 3b. It was found that his threshold to lightimproved, although not as much as the vitamin A deficient individualwhose chart is shown in Fig. 2 or the substantially normal individualwhose chart is shown in F 1.

In making the test it is essential to first dark adapt the eye as shownin the charts until the visual purple in the retina of the eye isapproximately constant. About ten minutes or longer is required for thispurpose. After the bleaching operation the threshold to light must bequickly determined, preferably within twenty seconds. This is a verysignificant reading because as visual purple is being" bleached from theeye it is constantly being regenerated from the system and the rate ofregeneration is particularly rapid after the bleaching operation. -Toprovide accurate results, it is also essential that the bleaching lightand the test light shall have the same energy distribution in thevisible spectrum because it has been found by tests that the visualpurple may be bleached with light having a particular energydistribution while it is being regenerated after bein bleached withlight having a different energy distribution. or instance, the bleachinglight may be composed of rays of such wave lengths that the color of thebleaching light is red, or' the rays may be of such wave lengths thatthe color of the bleaching light is green or blue. If .the bleaching iseffected 'by light having a particular energy distribution, however, thetest to determine the end point to visible perception of'the lightadapted eye must be made with light having the same energy distribution.While any desired filter may be employed to provide a bleaching lightand a test light having the same energy distribution, a daylight filteris preferably utilized for both the bleaching and the test light becauseit provides a more even energy distribution over the spectrum.

The significance of applicants improved test and the error incidenttoprior theories will be readily apparent from Fig. 4 of the drawings inwhich the numeral light of an individual as he came into the testingroom and the numeral his threshold to light after he had been darkadapted for aperiod of ten minutes. The visual purple in the retina ofthe eye of this individual was then bleached for one minute and the timerequired to regenerate sufllcient visual purple in the retina of the eyeto see light or 0.1 millifoot candle power intensity of the same qualityor energy distribution over the visible spectrum was determined asindicated by the numeral 1. His eyes were again bleached for periods oftwo minutes, three minutes, four minutes and five and onehalf minutes,respectively, with light of thesame intensity, the eye being darkadapted after each bleaching operation, and the time required toregenerate sufhcient visual purple in the retina of the eye to see lightof 0.1 candle power after each bleaching operation was determinedrasindicated by the numerals la, lb, and id. This chart indicates clearlythat previous light environment is an essential factor in determiningthe rate at which visual purple is regenerated inthe retina of the eyeduring dark adaptation, but that during the bleaching process anequilibrium is soon established between the amount of visual purplewhich is bleached in the retina of the eye and the amount which isregenerated. This is shown b the amount of light required for visibleperception after bleaching periods of the time specified, as indicatedby the points 5, ea, 5b, 6c and so.

An apparatus for performing the test shown in charts l to 3, inclusive,is disclosed in Figs. 5 to 13, inclusive. The apparatus comprises asuitable housing or casing which serves to enclose, contain and supportwithin it the working parts of the apparatus as well as to supportcontrolling and indicating parts accessible or visible from itsexterior. The casing shown in the drawings comprises a main body portion3 having a chamber a to one end of-which body is attached a tubular neckformed of members it and ii, its outer end being provided with anopening l2 through which the patient looks into the neck chamber is andthrough a luminous translucent opalescent glass wall or window itforming a dividing wall between chambers Q and i3 and fixedly mounted inthe casing. This luminous wall of course always lies a chosen distancefrom the eyes, determined by the length of the neck it, it and-isuniformly illuminated by a light suitably produced within the chamber e,either by a test light for moderately illuminating the luminous wall andthereby illuminating the lighted spots a, b, c on a target is suitablymounted in a manner to be movable into or out of operative positioncrosswise of the chamber it between the eyes of the patient and theluminous window, or 'a bright light, the effect of which is applieddirectly to the eyes of the patient by his direct viewof the fullareaofthe luminous window.

With this apparatus and after the eye has been dark adapted for apredetermined period of time, preferably for five to ten minutes orlonger, the

4 designates the threshold to.

when a shutter or ployed and greater accuracy is provided because targetis swung into operative position shown in dotted lines and the intensityof its illumination is adjusted or regulated to determine the first endpoint or the threshold to light after which the target is swung intoinoperative position and the eyes of the patient are subjected to theapplication of bright lights to the luminous window for a predeterminedtime., After the lapse of the predetermined or standard period, thetarget is again swung into operative position and the threshold to lightof the light adapted eye is quickly determined.

The target l5 may be of any suitable form for the purpose, such as aplate l1 having apertures in theform of the usual quincunx, and ismovable either to idle or operative position as shown re-' spectively infull and dotted lines in Fig. 6 of the drawings, the plate l1 beingentirely opaque except at the groups of spots a, b, 0, through whichlight passes from the source to the eye of the observer. In the formshown, the plate i1 is hinged at Ila at one side of the neck chamher andin operative positionlies against the shoulder Nb of the neck. It may beconveniently operated by a link l8 connecting it to a platelike arm I8amounted on a shaft IS in the wall of the casing and provided with anoperating knob or handle Mo on its outer end. The several spots a, b and0 may all be alike, such as round, or they may differ in shape todistinguish them from each other, such as by giving them the outlines ofobjects familiar and interesting to children, or, as shownconventionally, triangular in group a, round in group b and square ingroup 0.

Suitable means is provided for varying the intensity of illumination ofthe spots on the diaphragm, not only to difierentiate the illuminationof spots a, b, c, from each other, as is usual, but also tosimultaneously vary the absolute intensity of the illumination of allspots. Difierentiation between spots may be secured, for example, byattaching a translucent membrane 2@ to the plate it by any suitablemeans, such as lugs 23a, and interposing a difierent number of layers ofpaper between the translucent membrane and the various apertures in theplate ii in such a manner that spots a will appearbrighter than spot 27,and spot "b brighter than spots 0. This feature is shown in Fig. 12,where spots a are covered with one layer M of trans- No. 129,044, filedon March 4, 1937, or by varying the amount of current supplied tothetest light which illuminates the window during determination of thevisible perceptionto light. A rheostat or potentiometer is preferablyemployed for varying the quantity of light because it is inexpensive andcan be generally used. By utilizing a, rheostat or potentiometer, awider range of readings may also be obtained than diaphragm method isemthe light is not concentrated on a particular spot.

The use of a rheostat or potentiometer in varying the quantity of light,however, introduces the factor of changes in the quality of test lightbecause of differences in filament temperature at different voltages. Tominimize this error, the instrument is calibrated by means of aphotocell which is placed across the neck piece l2 and is screened by afilter having a similar range of sensitivity as the human eye. By thismethod the proportion of light from the test light which actuallyreaches the eye at a certain reading on the scale 2% may be determinedin millifoot candles and as the intensity of the light is raised orlowered by varying the 4 amount of resistance in series with the light,

all

the number of millifoot candles of light reaching the eye may bedetermined at any scale reading.

The test light is also preferably provided with a daylight filter toprovide a light having a more even energy distribution over the visiblespectrum than artificial light. The use of a daylight filter isparticularly desirable when a rheostat or potentiometer is employed tovary the intensity of the light because when an ordinary Mazda lamp isemployed, variation in the voltage causes a variation in the quality ofthe light and the daylight filter partially corrects for emissionchanges in the filament.

Bright illumination for bleaching the patients eyes is secured byutilizing a source of light which is suflicient' to deplete visualpurple in the retina of the dark adapted eye faster than it isregenerated. It is essential that the bleaching light be of the samequality; in other words, that it shall have the same energy distributionin the visible spectrum as that utilized in the test light. Any suitablemeans may be provided .to filter the bleaching light and the test lightto provide light for bleaching and for testing which is of the samequality. For instance, both the bleaching light and the test light maybe passed through a suitable filter, such as a daylight or Wrattenfilter which may be located in any desired position between the sourcesof light and the target, such as in proximity to the dividing plate H4,or the opalescent plate I! may be formed of material that filters outrays of light from the difierent sources which vary from each other inquality or energy distribution in the ,visible spectrum.

As shown in the drawings, a 110 volt, watt daylight bulb 2! is utilizedfor bleaching which is mounted in a socket Mo on asuitable bracket 22which is attached to a panel 23, said lamp being in a circuit '24 from.the leads L1, L2 as shown in Fig. 13 of the drawings. Also included inthis circuit is a potentiometer 25 and connected in shunt with the lampis a voltmeter 26. Circuit 24 is in parallel, through a double poledouble throw switch 2'! with a circuit 28, said circuitalso'including-potentiometers 29 and 30 and the test light 3| fordetermining the end mm to visible perception, the test light 3| being inunted in a housing provided with a daylight filter 3lb. When circuit 28isconnected to leads L1, L2, the voltmeter 26 is connected in shunt withthe test light 3i and the potentiometer 29. It will thus be apparentthat by varying the resistance of potentiometer 25 when the bright lightis in the circuit the voltage across the terminals of the bright lampmay be varied to correspond to the voltage at which'the instrument wascalibrated and in like manner by varying the resistance of to correspondto that'at which the instrument was calibrated.

Sincemeans must be provided for determining the end point to visibleperception promptly after the visual purple has been bleached from theeye .in our improved instrument the knob i9a which moves the plate H toeither of its two positions is utilized to actuate switch 21.

In the arrangement shown said switch is of the toggle type mounted onthe inner surface of the wall of the casing with its operating arm 3lalying in' a recess 32 between the edges of two por-' tions 33, 34 of thearm I 8a, so that rotationjof shaft IS in one direction simultaneouslymoves the diaphragm to its idle position and throws switch 21 to itsposition energizing circuit 24,

' whereas movement of shaft [9 in the opposite direction movesthediaphragm to its operative position and switch 21 to its positionenergizing circuit 28. The potentiometers 30, 25 and 29 are mounted inchamber 1 on one of the walls of the casing, the operating shafts ofwhich extend through the wall and are provided on its opposite side withoperating knobs '36, 31 and 38 respectively, the shaft for rheostat 29being provided with a scale 20b for the purpose previously de-v scribed.

For convenience in replacement of lamps and adjustment orv repair of theapparatus, the main body of the casing is provided with a removablebottom 39 while the two members 8 and 9 of the neck are detachablybolted or otherwise connected toeach other and to the main casing body.Also preferably the outer end of the neck or the edge of the outeropening is specially formed to fit the forehead and nose, such as bycurving the edges of the opening as at 40 and providing it with a noserecess 4|, and the edge of said opening may be shielded with a removablerubber bead 42 not only to secure a more comfortable fit, but also amore effective seal against light leakage.

- Withthis apparatus, the operator, after connecting the device to asuitable source of current to the preparation position, in which circuit28 is closed and the test lamp 3| is illuminated. This step is effectedwhen the patient enters the testing room. Target I5 is in operativeposition, the voltage of the machine is set at the point at which it wascalibrated by means of resistor 30 and the end point to visibleperceptionis determined by operating knob 36 in a .direction whichvaries potentiometer 29 and thus the intensity of test light 3| untilvonly three of the spots, namely, spots a and b are visible. After theend point to visible perception has been determined the eye is darkadapted for a predetermined period of time, say ten minutes, and the endpoint to visible perception is again determined. The diaphragm l1 isthen moved to idle position, the bright light is turned on and thepatient applies his eyes to the opening in the end of the neck and looksdirectly at "the illuminated translucent wall IQ for a nated. The effectof this operation is to deplete circuit 28 and illuminates test lamp 3|.

the visual purple in the retina of the eye faster than it isregenerated.

Next, the operator turns the knob H911 to its other position and therebysimultaneously moves the five-point diaphragm to its operative position,opens circuit 24, turns oil! lamp 2|, closes Immediately anotherdetermination is made of the end point to visible perception. The eyesof the patient are then dark adapted for a standard period of time, sayfor three minutes, and the end point to visible perception is againdetermined. It may also be determined after five and ten minutes,respectively, as shown in the charts. Tests may also be made at one ormore intervals during the dark adaptation test prior to the bleachingoperation. The tests made during dark adaptation while not essential indetermining the vitamin A status of the patient gives valuableinformation to the eye specialist with respect to the dark I adaptedthreshold.

The test may be modified to a certain extent and significant resultsstill obtained. For instance, the eye may be dark adapted for a sumcientlength of time after the patient enters the room to permit the visualpurple in the retina of the eye to become substantially constant. Thedark adapted eye may then be bleached and the end point to visibleperception quickly obtained after the bleaching process with a testlight'of the same quality as the bleaching light. As indicated in Fig.13 a small light' wb may-be connected in series from leads L1, L2 forreading the scale 2% in case the test is made in a dimly lighted or darkroom.

A modification of our improved instrument is shown in Figs. 14 to 20,inclusive, of the drawings. While our improved test may be made with themodified instrument, the test obviously will not be as significant aswhen the instrument shown in Figs. to 13, inclusive, is employed. Fairlyaccurate results, however, can be obtained and ii a patient orindividual is deficient in vitamin A, this fact may be determined.

As illustrated, the instrument includes a casing d5 provided with achamber 6'? containing a pinrality of bulbs 48 mounted upon a panel 29extending across the chamber and beyond which is a voltmeter 5d and apotentiometer dimounted upon the end wall of the casing. The casing 66is extended to form a neck portion 52 through which a patient looks intothe neck chamber and to the luminous translucent opalescent wall orwindow 56, forming a dividing wall between thechambers 53 and 41. Theluminous wall always lies at a standard distance from the eye,determined by the neck 52 and is uniformly illuminated by the lightproduced within the chamber. To provide means for determining the endpoint of visible perception, a target 55 provided with a plurality ofapertures 56 is movable across the neck portion 52 from the idleposition shown in full lines to operative position shown in dotted linesin Fig. 14. The intensity of the apertures may be varied by any suitablemeans, such as by placing layers of translucent paper of differentthickness over the apertures, or varying the num-- ber of layers overthe different apertures. shown in the drawings, the apertures are in theform of a clock and translucent layers of paper of varying thickness areinterposed between two translucent glass panels 51 and 5B in front ofthe difierent apertures in such a manner that the quantity of lightvaries and preferably, light of' of numerals, as illustrated in Fig. 20by the numetal 59. The target 55 may swing to idle or operative positionby any suitable means, such as a shaft 60 iournaled in the casing towhich the target is secured and which is operated by a handle iii. Ifdesired the instrument may be mounted upon a suitable base 62 whichisprovided with'a member 63 for adjusting its inclination and while lightof any desired quality may be employed, daylight bulbsare preferablyutilized.

In making the test with this form of instrument, the patients eyes arefirst dark adapted for a period of from approximately five to tenminutes or longer. The target 54 is placed in position and the patientlooking through neck piece 52 is requested to state the highest numeralwhich he can see. The target is then swung to idle position and thevisual purple in the patients eyes is bleached for a predeterminedperiod of time, say three minutes, by means of light from the daylightbulbs; The target is then swung to operative position and the patient isagain requested to specify the particular number which he can see. Theeye is then dark adapted and a plurality of tests are made on the darkadapted ,eye as specified in the previous example. Before making thetest the instrument is preferably calibrated inthe method previouslydescribed and for this, purpose potentiometer 56 is connected in seriesand voltmeter Ed is connected in shunt with the lamps in the circuitfrom lines L1, Lo.

'From the foregoing description it will be seen that we have provided animproved process of determining the vitamin A status of individualswhich is not only effective in testing night blindness caused by vitaminA deficiency but which is also efiective in indicating the status'otindividuals or patients who are only mildly deficient in that vitamin.

It will also be apparent that we have provided an improved process ofdetermining-the rate at in which thevitamin A status of an individualvmay be indicated.

What we claim is:

1. An eye testing apparatus of the class described, comprising a casinghaving a body portion closed against the entrance or exit of light andan eye opening which is adapted to receive light rays which emanate fromwithin said body portion, re; source of bright illumination mounted insaid body portion in'optical alinement with the eye opening, a source ofdim illumination arranged in said body portion, the source of brightillumination having suflicient light energy to provide rays oisuflicient intensity to lightadapt a dark adapted eye placed at saidopening, a movable target mounted in said casing between saidilluminating sources and said eye-opening, said target having opaqueportions and translucent portions of varying degrees of translucencyproviding test object means adapted to be viewed throughthesaid eyeopening, said target being movable to an idle position in whichunobstructed rays of light pass from the source of bright illuminationto the eye opening or to an operative light threshold determiningposition inwhich all light rays which might pass to an eye at saidopening are-intercepted except light rays which pass through translucentportions of said target, the dim light source being arranged toilluminate said translucent portions when the target is in its operativeposition, means whereby said target may be moved either to its idle orto its operative position, means for rendering the source of brightillumination effective when said target is in idle position, meanswhereby the source of bright illumination may be made ineffective andthe dim source of illumination made efiective when the target is inoperative position,

and means for regulating the intensity of the dim source ofillumination.

2. Apparatus as specified in claim 1, provided with filtering meansarranged between both sourcescof illumination and said'eye opening toprovide light of substantially the same energy distribution in thevisible spectrum for both light adapting and for determining the lightthreshold.

3. Apparatus of the class described in claim 1, in' which the source ofdim illumination is energize'd by an electric circuit, and including apotentiometer associated with said circuit for enabling the intensity ofillumination from the dim source to be quickly adjusted manually to anend point determining value with relation to the relative translucenciesof different translucent portions of said target.

WILLIAM O. FROHRING. HOWARD F. SEZIBERT. JOSEPH M. TABOR.

